1. Field of the Invention
This invention relates to an image processing apparatus and method, and more particularly, to an image processing apparatus and method for performing color processing, and to a recording medium.
2. Description of the Related Art
FIG. 1 is a flowchart illustrating processing when printing on a color printer a color image which is displayed on a CRT (cathode-ray tube). In FIG. 1, processing for converting R, G and B image data for CRT display into C, M, Y and K image data corresponding to inks of four colors (cyan (C), magenta (M), yellow (Y) and black (K)) for printing on a color printer.
By performing luminance-to-density conversion in step S102 for R, G and B image data (Rin, Gin and Bin) input in step S101, C, M and Y image data (Cin, Min and Yin) are obtained. Inks of three primary colors, cyan, magenta and yellow, do not have ideal colors, and also have other color components (unnecessary components). In order to compensate for the influence by these unnecessary components, color correction is performed for the C, M and Y image data in step S103. For example, a masking method is known as a method for such color correction.
C, M and Y image data (C1, M1 and Y1) subjected to color correction are converted, in step S104, into C, M, Y and K image data (C2, M2, Y2 and K2) according to black-generation processing. In general, the minimum value of the C, M and Y image data is made to be an achromatic-color component and is used as black-image data K (black generation). By subtracting the value of K from the respective original C, M and Y image data (removal of under color), C, M, Y and K image data (C2, M2, Y2 and K) are obtained. Then, in step S105, density correction desired by the user is performed. Then, in step S106, output correction is performed, and obtained output data (Cout, Mout, Yout and Kout) are transmitted to a color printer.
FIG. 2 is a flowchart also illustrating processing when printing on a color printer a color image which is displayed on a CRT. In the flowchart of FIG. 2, however, black-generation processing (step S104) is performed before performing color correction (step S103).
The above-described techniques, however, have the following problems.
In the processing shown in FIG. 1, the minimum value of C, M and Y image data after performing luminance-to-density conversion is made to be a black component. When, for example, the C component from among the C, M and Y image data has a minimum value, if the C component is included in unnecessary components of inks corresponding to the remaining M and Y components, achromatic-color components are produced due to the unnecessary components, thereby degrading saturation in the entire image.
In order to prevent degradation in saturation by removing influence by the unnecessary components of inks in the processing shown in FIG. 1, it is necessary to perform conversion in which the unnecessary components are taken into consideration in the luminance-to-density conversion in step S102. For that purpose, it is necessary to form a large number of color patches, to measure the densities of the respective color patches, and to calculate a conversion table or the like from the results of the measurement. This requires a significant number of operations. Furthermore, when performing luminance-to-density conversion in which unnecessary components in inks are taken into consideration, it is difficult to make achromatic colors represented by C=M=Y to be achromatic colors after the correction. That is, it is very difficult to prevent degradation in saturation in the entire image while maintaining reproducibility for achromatic colors (gray balance).
When performing color correction after performing black-generation processing shown in FIG. 2, also, influence of unnecessary components in inks is not removed, thereby degrading saturation in the entire image.